Many types of non-volatile memory devices have been researched and developed. A magnetic random access memory (MRAM) has been developed as one of non-volatile memory devices which features `hardness to radiation`. A memory cell in the MRAM has basically a word line, a sense line, and a portion of magnetoresistive material which is typically giant magnetoresistive (GMR) material, all of which are formed on a semiconductor substrate. The word line is fabricated perpendicular to the sense line, and the portion of GMR material is placed at a crossing point of the word line and the sense line. The portion of GMR material stores information as states "0" and "1" which correspond to directions of magnetic vectors in GMR material. The states in the memory cell are stored and changed by applying a magnetic field over a predetermined threshold level to the portion of GMR material. A magnetic field generated by a word current is combined with a magnetic field generated by a sense current, and the total magnetic field is applied to the portion of the GMR material to read and store the states. The states stored in the memory cell are read out by sensing the voltage difference at the output terminal of the sense line because resistance of GMR material is alternated in accordance with directions of magnetic vectors.
The conventional MRAM integrates a plurality of memory cells on the semiconductor substrate. Since the memory cells are arranged on a plane and the memory cell requires a certain area, the number of memory cells integrated on the substrate is limited.
Accordingly, it is a purpose of the present invention to provide a new and improved magnetic random access memory with a high density of memory cells.
It is a further purpose of the present invention to provide a new and improved magnetic random access memory with high density and a low power consumption.